In many instances, titanium carbide is used in the form of a composite sintered body. Particularly, a titanium carbide and alumina composite sintered body is used in various applications, such as cutting tools, wear-resistant parts and thin-film magnetic head substrates, based on its excellent characteristics, such as high-temperature strength, heat resistance, wear resistance and chemical resistance.
On the other hand, titanium carbide has a disadvantage of poor sinterability. If titanium carbide is used as a part of a composite material to prepare a sintered body, the sintered body is highly likely to have residual voids or pores. Thus, it is necessary to add a sintering aid for increasing the degree of sintering, which leads to a problem about deterioration in strength of the sintered body.
As means for solving this problem, it is effective to decrease particle sizes of titanium carbide powders in order to improve the sinterability. If a titanium carbide powder is formed to have smaller particle sizes, it will have enhanced sinterability and can be sintered at lower temperature. This allows a grain growth of ceramics to be effectively suppressed during a process of preparing a composite sintered body in combination with ceramic powder. It is known that, when a particle size of a titanium carbide powder is reduced to 100 nm or less, the above effect can be significantly enhanced, and additionally the titanium carbide powder exhibits an excellent dispersion strengthening effect in a sintered body.
A titanium carbide powder is widely used as an addition for improving high-temperature hardness and wear-resistant characteristic in WC/Co-based hard metal cutting tools, or as an initial raw material for cermet tools, rolls and dies, in the form of a composite material combined with a metal powder, such as a Ni powder.
Recently, in view of particle-size reduction (i.e., micronization or nanonization) of titanium carbide powders which allows a tool to have higher hardness, higher transverse rupture strength and enhanced wear resistance, a particle-size reduction techniques for titanium carbide powders become a key challenge.
Heretofore, a titanium carbide powder has been produced by a process of subjecting a mixed powder of titanium dioxide (TiO2) and carbon to a heat treatment in a non-oxidizing atmosphere at a high temperature of about 1500° C. to reduce/carbonize the mixed powder, or by a direct carburization process using Ti and TiH2.
TiC powder produced by the above conventional processes have large particle sizes of 1 to 10 μm, and therefore the particle sizes are reduced by ball milling. However, it is difficult to reduce a maximum particle size to 0.5 μm or less. Moreover, grinding media are inevitably mixed in the powder to cause deterioration in powder quality.
With a view to solving these problems, the following Patent Publication 1 discloses a technique of putting a mixed solution of titanium tetrachloride (TiCl4) and carbon chloride into a closed container containing molten magnesium (Mg) metal under an inert atmosphere, vacuum-separating excess liquid Mg and magnesium chloride (MgCl2) remaining after a magnesium reduction reaction, and collecting a TiC-base composite from the closed container after the vacuum separation of the liquid Mg and the MgCl2.
Based on the technique disclosed in the Patent Publication 1, a titanium carbide powder can be synthesized at a temperature of 900 to 1000° C. which is lower than ever before. In addition, the obtained titanium carbide powder has a fine particle size of 50 nm, and contains free carbon in a small amount of 0.2 weight %, with a titanium-carbide crystal structure having a lattice constant of 4.3267 Å which is close to a theoretical value.
However, the above titanium carbide powder involves a problem about a large content of impurities, specifically, 0.3 to 0.8 wt % of Mg, 0.1 to 0.3 wt % of Cl and 0.1 to 0.6 wt % of Fe.
The following Patent Publication 2 discloses a technique of: the mixture of water-soluble salt which contains a titanium, one of a metatitanic acid [TiO(OH)2] slurry or an ultrafine titanium oxide powder, and solution which dissolved water-soluble metal salt which contains a transition metal in water were prepared as a mixed raw material; spray-drying the mixed raw material to obtain a precursor powder; subjecting the precursor powder to a heat treatment to form an ultrafine Ti-transition metal composite oxide powder; mixing nanosized carbon particles with the ultrafine Ti-transition metal composite oxide powder; drying the mixture to obtain a composite oxide powder; subjecting the composite oxide powder to a reduction treatment in a non-oxidizing atmosphere and a carburization heat treatment at 1200 to 1350° C. to produce a TiC—Co composite powder in which a titanium carbide crystal has grain sizes of 35 to 81 nm.
Although the technique disclosed in the Patent Publication 2 is designed to set a content of transition metal at 1 wt % or more so that the reduction/carburization heat treatment can be performed at a temperature of 1350° C. or less to obtain an ultrafine powder, it is difficult to produce only a highly-pure fine titanium carbide powder in a non-composite form.
Meanwhile, a synthesis of titanium carbide using a liquid phase synthesis has advantages of being able to stably obtain a fine carbide, and easily mix with other component.
Further, titanium alkoxide used as a titanium source provides an advantage of allowing a titanium carbide powder with an extremely small amount of other mixed metal component to be obtained at relatively low cost.
However titanium carbide powder got by the liquid-phase method reported until now contained free carbon over several wt % or more as impurities, when it was used as a sintering raw material, the free carbon would disturb sintering to cause a problem about difficulty in obtaining a dense sintered body.
For example, the following Non-Patent Publication 1 discloses a technique of mixing titanium isopropoxide with several types of dicarboxylic acids having different chelation properties drying the mixture, and subjecting the dried product to a heat treatment in an argon atmosphere containing 0 to 10% of hydrogen to obtain a titanium carbide powder. However, the obtained titanium carbide powder contains free carbon in an amount of 4.2 wt % or more.
As above, no mass production technology has been established that is capable of producing a titanium carbide powder which has a maximum particle size of 100 nm or less, and contains free carbon in an amount of 0.5 wt % or less and metals except titanium in a small amount.
In a process of preparing a composite sintered body of titanium carbide and other ceramics including alumina, if a titanium carbide powder as a raw material has a smaller particle size, it is more likely to aggregate and thereby cause difficulty in obtaining a sintered body with titanium carbide grains homogeneously dispersed therein.
As a technique of solving this problem, in a process of preparing a titanium carbide-dispersed ceramics sintered body, powder particles each having a so-called core-shell structure where a surface of each ceramics particle is covered with titanium carbide particles, are effective in preventing aggregation of a titanium carbide powder so as to obtain the sintered body with a homogenous structure. The core-shell particles are also effective in suppressing grain growth of ceramics during sintering.
The following Patent Publication 3 discloses one production method for such a composite powder. The method disclosed in the Patent Publication 3 comprises synthesizing powder particles with a core-shell structure where a TiC thin film is formed on a surface of each alumina particle by a CVD (Chemical Vapor Deposition) process, and sintering the powder to obtain a sintered body with titanium carbide grains homogeneously dispersed therein. However, the CVD process is originally a batch production process to be performed in a vacuum apparatus, which is unsuitable for mass production and costly.                [Patent Publication 1] JP 2005-047739A        [Patent Publication 2] JP 2004-323968A        [Patent Publication 3] JP 05-270820A        [Non-Patent Publication 1] Tom Gallo, Carl Greco, Claude Peterson, Frank Cambira and Johst Burk, Azko Chemicals Inc., Mat. Res. Soc. Symp. Proc. Vol. 271, 1992, pp 887-892        